In the manufacture of continuous glass fibers, glass batch ingredients are melted in furnaces. The molten glass resulting from the melting of the glass batch mixtures is then fed into elongated forehearths of various sizes and shapes. The molten glass flowing in the forehearths is removed at several locations positioned along the length of the forehearths from fiber forming bushings connected to the forehearth. In a typical fiber glass manufacturing operation, a glass level of predetermined height is maintained in the forehearth being fed by the glass furnace. These forehearths are normally constructed of ceramic materials resistant to molten glass attack and are frequently heated by combustion gases to maintain the molten glass during its passage through the forehearth at a given temperature. In some instances, electrodes can be used to supply all or a part of the energy as electricity to the molten glass in the forehearth to maintain the temperature of the glass at a predetermined constant.
The molten glass, as stated above, is removed from the forehearth at bushing positions located along the forehearth. In a typical position in which a bushing is located a ceramic block, called a flow block, is positioned above the bushing and typically is provided with a cutout section sloping downwardly from the forehearth bottom. This sloped opening permits the molten glass to flow from the forehearth floor to the fiber glass bushing located beneath the flow block. The flow block is normally associated with a bushing block located directly below it and to which is attached a precious metal, fiber glass forming bushing. The bushing block, like the flow block, is typically a ceramic structure with an opening adapted to support the bushings attached to it. The bushings are generally rectangular containers open at the top to receive molten glass and made of a precious metal alloy, typically platinum-rhodium alloy. While rectangular shapes are normally employed other configurations have been used. Thus, circular bushings have been used as well as square and tubular bushings.
Whatever the shapes, the bushings usually contain a plurality of orifices on the bottom thereof, 200 to 2000 or more being typical. The orifices are arranged usually in rows and vary in diameter and number from bushing to bushing depending on the product strand being made by a given bushing. The fiber glass bushing itself is mounted, typically in a cast iron frame. The frame has within its confines a castable ceramic surrounding the bushing metal to electrically isolate it from the frame material since during operation the bushing is electrically heated to maintain a given, uniform temperature therein. A typical material for this use is described in detail in U.S. Pat. No. 3,164,457 at column 4, lines 30-37. The frame is bolted to the bushing block in the normal fashion. In U.S. Pat. No. 3,837,823 a general description of the mounting of a bushing in a bushing frame and bushing block is described in detail. This patent also shows in general an arrangement of a furnace or melter and the forehearth and bushing positions associated with it.
In the manufacture of glass fiber strands, it is common for a given furnace feeding a forehearth to have positioned on that forehearth as many as 40 to 80 bushings or more. These bushings are subject to frequent replacement due to changing demands for products being made therefrom in the marketplace, as well as for repairs caused by mechanical failures such as leaks or cracks occurring in the bushings. Thus, for example, a given forehearth can be operating with a plurality of bushings which are manufacturing a G75's strand. This would mean that the forehearth is operating with 400 hole bushings which manufacture strands having 400 glass filaments in each strand and in which each filament has a diameter of 0.00036 to 0.00039 inches. During the course of that manufacture, which can occur over several months, the demand may decrease for G75 fibers. At the same time an increase in demand for K15 fibers may occur. When this happens, bushings must be changed to accommodate the changing requirements of the marketplace by replacing the bushings which manufacture the declining market strand with bushings capable of making the strand which is increasing in demand.
In order to change a fiber glass bushing on an operating forehearth, considerable difficulties are encountered in that the bushing is mounted to ceramic blocks which are in communication with molten glass contained in and continuously flowing in a forehearth. Thus, in order to remove a bushing from its position on a forehearth, the bushing first has to have its electrical supply cut off. Once this is done, the glass contained in the bushing itself is allowed to cool until it solidifies. The bushing and the associated ceramic blocks located above it must be chilled also to insure that the molten glass in the bushing block above the bushing itself has solidified prior to removal of the bushing from its bushing connection to the bushing block. When that glass, i.e., the glass in the bushing block is solidified, the bushing may then be chiseled away from the bushing block.
The chilling of bushing block and bushing is usually carried out by directing water onto the surfaces thereof for considerable periods of time. This causes thermal shock to the glass flowing in the forehearth in the vicinity of the position, which frequently effects the bushings on the forehearth located near the bushing being removed. The chiseling of the glass at the bushing-bushing block interface causes physical shock to the forehearth refractory in the vicinity of the position. These shocks cause a mechanical disturbance to the glass in the forehearth and contamination can occur in the glass flowing in the forehearth. In addition, the shock cooling of such large masses of glass and the subsequent mechanical shocks which introduce physical as well as thermal shocks to the forehearth are time consuming as well as being deleterious to normal operations. Moreover, they also reduce the productive efficiency of the entire forehearth.
Because the above circumstances existent in the art today, a need persists in the art to provide it with a bushing assembly and method of removing a bushing from a forehearth which can overcome the shortcomings of the present state of the art. Such a method and assembly are needed so that in the normal operation of a manufacturing plant frequent bushing changes can be made with a minimal disturbance to the thermal environment of the forehearths to which the bushings are attached. A method and assembly also are needed to minimize physical shock and damage to the surrounding ceramics to which the bushings themselves are attached during operation when a bushing change is made.
In accordance with the instant invention, glass fiber forming apparatus and methods are provided which satisfy this requirement of the prior art. The construction of the apparatus and operation of the methods described are such that bushings may be removed from service during a fiber glass continuous strand operation rapidly and with minimum effect on the surrounding environment.